A copy-protection device for a semiconductor chip with a cover has a
resonant oscillating circuit whose elements are located on the chip
surface and within the cover. The semiconductor chip also has an
evaluation circuit connected to the oscillating circuit. The evaluation
circuit disables the semiconductor chip if the oscillating circuit is
detuned and thus stopped from resonating. This detuning of the oscillating
circuit occurs when the cover is removed, since the cover acts as a
dielectric for the capacitor. After the cover has been removed, it is
impossible to operate the semiconductor chip, in order to copy or analyze
it.

This is a continuation of copending International Application
PCT/DE97/02039, filed Sep. 11, 1997, which designated the United States.

Claims

We claim:

1. A semiconductor device, comprising:

a semiconductor chip having a coil and a capacitor defining an oscillating
circuit with a resonant frequency;

an electrically insulating layer forming a dielectric for said capacitor
and covering said semiconductor chip; and

an evaluation circuit connected to said oscillating circuit for detecting a
detuning of said oscillating circuit due to a change in said resonant
frequency caused by altering said electrically insulating layer, said
evaluation circuit disabling said semiconductor chip upon detecting the
detuning of said oscillating circuit.

2. The semiconductor device according to claim 1, wherein said
semiconductor chip has a top surface, at least said coil being formed as a
metallic conductor track over said top surface.

3. The semiconductor device according to claim 2, wherein said metallic
conductor track includes parallel conductor track sections, said capacitor
being formed by said parallel conductor track sections.

4. The semiconductor device according to claim 1, which further comprises
at least a further oscillating circuit coupled to said oscillating circuit
for oscillating.

5. The semiconductor device according to claim 1, wherein said
semiconductor chip has a surface, said coil spanning said surface
substantially entirely.

6. The semiconductor device according to claim 2, wherein said capacitor
substantially entirely occupies said top surface at free surface areas
thereof.

7. The semiconductor device according to claim 1, wherein said evaluation
circuit, after application of said electrically insulating layer,
ascertains said resonant frequency of said oscillating circuit and drives
said oscillating circuit during an operation of said semiconductor chip.

8. The semiconductor device according to claim 1, wherein said evaluation
circuit drives said oscillating circuit at a preset frequency.

9. The semiconductor device according to claim 1, wherein said electrically
insulating layer forms a cover for said semiconductor chip, said
evaluation circuit detecting the detuning of said oscillating circuit due
to a removal of said cover.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a semiconductor chip having at least one capacitor
and an electrically insulating layer that forms a dielectric for the
capacitor, as well as an evaluation circuit which disables the
semiconductor chip if the layer is removed.

Such a semiconductor chip is known from U.S. Pat. No. 4,868,489. In that
patent, the dielectric property of the passivation layer of the
semiconductor chip is used in order to make it possible to detect a
removal of this passivation layer. However, this detection is carried out
only by a direct measurement of the capacitance of two neighboring
metallic surfaces.

Published German Patent Application DE 40 18 688 A1 discloses detecting the
capacitance, inductance and resistance of a conducting surface, which is
applied to the semiconductor chip surface and covers the underlying
circuits, in order to make it possible to ascertain when this protective
layer has been removed. This involves however determining the capacitance
relative to the underlying layers. Furthermore, the inductance of such a
surface is very small, so that an evaluation requires circuits which are
highly sensitive and therefore elaborate and expensive.

The costs involved in developing an integrated circuit, hereinafter
referred to as semiconductor chip, are known to be very high, while
production costs, in particular in very large production runs, may be
comparatively low. Semiconductor chips are therefore exposed to the risk
of being analyzed and copied without permission. For this reason, as
already indicated above, various measures have been developed which are
intended to prevent unauthorized analyzing of the way in which a
semiconductor chip functions, as well as its structure.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a semiconductor
chip with an improved protection against analyzing which overcomes the
above-mentioned disadvantages of the heretofore-known semiconductor chips
of this general type.

With the foregoing and other objects in view there is provided, in
accordance with the invention, a semiconductor device, comprising:

a semiconductor chip having a coil and a capacitor which define an
oscillating circuit with a resonant frequency;

an electrically insulating layer forming a dielectric for the capacitor and
covering the semiconductor chip; and

an evaluation circuit connected to the oscillating circuit for detecting a
detuning of the oscillating circuit due to a change in the resonant
frequency caused by altering the electrically insulating layer, the
evaluation circuit disabling the semiconductor chip upon detecting the
detuning of the oscillating circuit.

In accordance with another feature of the invention, the semiconductor chip
has a top surface, at least the coil being formed as a metallic conductor
track over the top surface.

In accordance with yet another feature of the invention, the metallic
conductor track includes parallel conductor track sections, the capacitor
being formed by the parallel conductor track sections.

In accordance with a further feature of the invention, a plurality of
oscillating circuits are coupled for oscillating.

In accordance with yet a further feature of the invention, the
semiconductor chip has a surface and the coil spans the surface
substantially entirely.

In accordance with another feature of the invention, the capacitor
substantially entirely occupies the top surface at free surface areas
thereof.

In accordance with yet another feature of the invention, the evaluation
circuit, after the electrically insulating layer is applied, ascertains
the resonant frequency of the oscillating circuit and drives the
oscillating circuit during an operation of the semiconductor chip.

In accordance with another feature of the invention, the evaluation circuit
drives the oscillating circuit at a preset frequency.

In accordance with yet another feature of the invention, the electrically
insulating layer forms a cover for the semiconductor chip, the evaluation
circuit detects the detuning of the oscillating circuit when the cover is
removed.

The object of the invention is in particular achieved by the semiconductor
chip having at least one inductor which, together with the capacitor,
forms a resonant oscillating circuit, so that the oscillating circuit is
detuned if the resonant frequency of the oscillating circuit changes due
to changing the semiconductor chip's cover which forms a layer of
dielectric for the capacitor.

A basic idea of the invention is therefore to produce an electrical
coupling with the cover, wherein the electrical coupling is needed for the
semiconductor chip to function. This coupling is achieved by the fact that
the cover forms a necessary element of an oscillating circuit, having an
inductor and a capacitor. The cover effects the dielectric constant of the
capacitor of the oscillating circuit. If the cover is tampered with, or if
the cover is removed, then the semiconductor chip can no longer be
operated on account of the detuning of the resonant circuit. However,
since operation of the semiconductor chip is a prerequisite for detailed
analyzing, reliable protection against analyzing and copying of the
semiconductor chip is therefore obtained.

If the cover is removed or modified, the oscillating properties of the
oscillating circuit also change. Either the resonant frequency is shifted
or the ability to oscillate is lost. In both cases, the oscillating
circuit is stopped from resonating and the evaluation circuit permanently
disables the semiconductor chip. Security is further increased if there
are a plurality of oscillating circuits, which are coupled to one another.
An attempt, after removing the cover, to fit a replacement circuit with
reconstruction of the tuning is doomed to failure as a result of this
measure.

According to a further embodiment of the invention, the evaluation circuit
is constructed in such a way that following the application of the cover
to conclude the production process of the semiconductor chip, the
evaluation circuit automatically ascertains the resonant frequency of the
oscillating circuit or circuits and drives them at resonance during the
operation of the semiconductor chip. The resonant frequency is determined
only once before the semiconductor chip is being used. After any removal
of the cover, it is impossible to retune the oscillating circuit or the
oscillating circuits as they were originally. In this embodiment, the
effect of fabrication tolerances on the tuning of the oscillating circuit
or the oscillating circuits is compensated or neutralized since the
evaluation circuit according to the invention ascertains the tuning when
the semiconductor chip and the cover are in a fully assembled state.

Alternatively, a high security can also be achieved if the evaluation
circuit is configured in such a way that it drives the oscillating circuit
or the oscillating circuits with a fixed frequency. The fixing of the
specific resonant frequency, or the tuning to a standard value by the
evaluation circuit, is carried out automatically and cannot be accessed
externally.

In principle, the inductor and the capacitor of the oscillating circuit or
oscillating circuits may be formed as metallic structures under a
hermetically sealing surface layer of the semiconductor chip. Particularly
simple production can, however, be achieved if the inductor(s) and
capacitor~s) are applied as metallic conductor tracks directly to the
surface layer of the chip. A capacitor will preferably be formed by
parallel conductor track sections of an inductor. This has the advantage
that the inductor and capacitor can be constructed as a simple geometrical
structure.

The sensitivity of the oscillating circuit is preferably increased by an
inductor and/or capacitor that spans or occupies the entire chip surface.

Other features which are considered as characteristic for the invention are
set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a
semiconductor chip with protection against analyzing, it is nevertheless
not intended to be limited to the details shown, since various
modifications and structural changes may be made therein without departing
from the spirit of the invention and within the scope and range of
equivalents of the claims.

The construction and method of operation of the invention, however,
together with additional objects and advantages thereof will be best
understood from the following description of specific embodiments when
read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a schematic partial sectional view of a
semiconductor chip shown in a transparently illustrated package.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the single FIGURE of the drawing in detail, there is seen
a semiconductor chip 1 located approximately at the center of a cuboid
package or housing 2 which is made of an electrically insulating plastic
compound and, with its upper area, forms a cover layer for the top side of
the semiconductor chip 1. A cover 4 made of dielectric material is
therefore formed over the chip surface 3. Directly on the chip surface 3,
a metallic structure 5 is applied which forms an inductor and a capacitor
of an oscillating circuit. The inductor is formed by a spiral conductor
track 6 with two end terminal points 7 and spans as much as possible of
the entire peripheral area of the chip surface 3. The capacitor is formed
by parallel dummy conductor track sections 8 of the inductor. The inductor
and the capacitor are electrically connected to further components of the
resonant oscillating circuit, which are integrated in the semiconductor
chip 1. The semiconductor chip 1 also contains an electronic evaluation
circuit 9 which is connected to the resonant oscillating circuit and which
disables the semiconductor chip if the resonant oscillating circuit does
not properly oscillate as it was intended to.

The dielectric plastic material of the cover 4 sets the value of the
capacitance of the capacitor of the resonant oscillating circuit. If the
cover 4 is removed, then the capacitance value of the capacitor changes as
well since the relative geometrical configuration of dielectric material
and capacitor is altered. The associated oscillating circuit is thus
stopped from resonating. This fact is ascertained by the evaluation
circuit 9 and irreversibly results in disabling the semiconductor chip 1.